Image display device

ABSTRACT

An image display device includes: a video light generation unit that generates video light modulated based on a video signal; a light diffraction unit (first diffraction optical element) that diffracts the video light emitted from the video light generation unit; a light scanning unit (light scanner) that spatially scans the video light; and a reflection unit that includes a light diffraction unit (second diffraction optical element) that diffracts video light scanned by the light scanning unit. A light reflection unit included in the light scanning unit is configured by the light diffraction unit. In the light diffraction unit, an interference fringe pitch is preferably constant. The light diffraction unit has portions in which interference fringe pitches are mutually different.

BACKGROUND

1. Technical Field

The present invention relates to an image display device.

2. Related Art

Head-mounted displays (HMDs) are known as display devices that directlyradiate lasers to retinas of pupils and cause users to view images.

Head-mounted displays generally include light-emitting devices that emitlight and scanning units that change light paths so that the emittedlight scans retinas of users. Such a head-mounted display enables a userto simultaneously view both of, for example, an outside scenery and animage depicted by the scanning unit.

For example, JP-A-2014-78022 discloses an image display device includinga light source, a scanning unit that scans parallel light emitted fromthe light source, and an optical device that relays and emits theparallel light scanned by the scanning unit toward eyes. Of these units,the optical device is disclosed which includes a light-guiding plateconfigured to propagate incident light by total reflection therein andthen emits the light, a first diffraction grating member that diffractsthe light so that the light incident on the light-guiding plate istotally reflected, and a second diffraction grating member thatdiffracts the light so that the light totally propagated by the totalreflection can be emitted from the light-guiding plate.

In the image display device disclosed in JP-A-2014-78022, the lightscanned by the scanning unit is configured to be incident on the firstdiffraction grating member. Since the light incident on the firstdiffraction grating member is scanned over a two-dimensional scanningrange with a constant area, it is necessary for the first diffractiongrating member to have a sufficient size so that the light can bereceived. As a result, the size of the image display device disclosed inJP-A-2014-78022 may be necessarily increased.

SUMMARY

An advantage of some aspects of the invention is to provide an imagedisplay device capable of suppressing deviation in a diffraction anglecaused in diffraction and performing high-quality display whilesuppressing an increase in the size of the device.

The advantage can be achieved by the invention described below.

An image display device according to an aspect of the inventionincludes: a video light generation unit that generates video lightmodulated based on a video signal; a first diffraction optical elementthat diffracts the video light emitted from the video light generationunit; a light scanner that includes a light reflection unit andspatially scans the video light by reflecting the video light in thelight reflection unit; and a second diffraction optical element thatdiffracts the incident video light when the video light scanned by thelight scanner is incident. The light reflection unit is configured bythe first diffraction optical element.

Accordingly, since the first diffraction element is small, an increasein the size of the device is suppressed and deviation in a diffractionangle caused in the diffraction can be suppressed. Therefore, it ispossible to obtain the image display device capable of performinghigh-quality display.

In the image display device according to the aspect of the invention, itis preferable that, in the second diffraction optical element, a surfaceshape on an incident side of the video light is a concave surface in adirection perpendicular to a diffraction grating of the seconddiffraction optical element.

With this configuration, since the second diffraction optical elementhas the equivalent function to a condensing lens, a function ofcondensing the video light toward the eye of the observer is enhanced.As a result, the observer can view the video with a large angle of viewand high quality.

In the image display device according to the aspect of the invention, itis preferable that the light scanner performs main scanning of the videolight in a first direction and performs sub-scanning of the video lightin a second direction orthogonal to the first direction, in the firstdiffraction optical element, a diffraction grating period is constant,and the second diffraction optical element has portions in which thediffraction grating period is mutually different from a diffractiongrating period on a scanning line of the main scanning passing through acenter of an amplitude of the sub-scanning of the video light incidenton the second diffraction optical element.

With this configuration, the video light projected to the seconddiffraction optical element while being scanned two-dimensionally can bediffracted to be incident on the eye of the observer in the seconddiffraction optical element. Therefore, the observer can view the videowith a large angle of view and high quality.

In the image display device according to the aspect of the invention, itis preferable that the diffraction grating period of the firstdiffraction optical element is between a maximum value and a minimumvalue of the diffraction grating period on the scanning line of the mainscanning pas sing through the center of the amplitude of thesub-scanning of the video light incident on the second diffractionoptical element.

With this configuration, the angle width of the diffraction anglesoccurring in diffraction of the first diffraction optical element can besufficiently offset in diffraction of the second diffraction opticalelement in the substantially entire region of the second diffractionoptical element.

In the image display device according to the aspect of the invention, itis preferable that the diffraction grating period of the firstdiffraction optical element is the same as a diffraction grating periodat a position of a center of an amplitude of the main scanning and onthe scanning line of the main scanning passing through the center of theamplitude of the sub-scanning of the video light incident on the seconddiffraction optical element.

With this configuration, the angle width of the diffraction anglesoccurring in the diffraction of the first diffraction optical elementcan be sufficiently offset in the diffraction of the second diffractionoptical element in the substantially entire region of the seconddiffraction optical element.

In the image display device according to the aspect of the invention, itis preferable that the diffraction grating period of the firstdiffraction optical element is the same as an average value of thediffraction grating period on the scanning line of the main scanningpassing through the center of the amplitude of the sub-scanning of thevideo light incident on the second diffraction optical element.

With this configuration, the angle width of the diffraction anglesoccurring in the diffraction of the first diffraction optical elementcan be sufficiently offset in the diffraction of the second diffractionoptical element in the substantially entire region of the seconddiffraction optical element.

In the image display device according to the aspect of the invention, itis preferable that an extension direction of the diffraction grating ofthe second diffraction optical element is orthogonal to the firstdirection.

With this configuration, the angle width of the diffraction anglesoccurring in the diffraction of the first diffraction optical elementcan be reliably offset in the diffraction of the second diffractionoptical element.

In the image display device according to the aspect of the invention, itis preferable that the light scanner includes a driving system thatreciprocates and rotates the light reflection unit, and when γ₀ is anamplitude of the reciprocation and the rotation, α is an angle formed byan optical axis of the video light and a normal line of the lightreflection unit when the light reflection unit does not rotate, and β₀is a minimum value of an angle formed by the light axis of the videolight incident on the light reflection unit when the light reflectionunit reciprocates and rotates and the light axis of the video lightemitted from the light reflection unit, a formula below is satisfied.

$\alpha < \frac{\beta_{0} - \gamma_{0}}{2}$

With this configuration, it is possible to reduce a probability at whichthe reflected light reflected from the surface of the first diffractionelement becomes stray light to be viewed. As a result, it is possible tosuppress deterioration in visibility of the video.

It is preferable that the image display device according to the aspectof the invention further includes a pupil expansion optical system thatis provided on a light path between the light scanner and the seconddiffraction optical element.

With this configuration, it is possible to expand a light flux width(cross-sectional area) of the video light, and thus improve thevisibility.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to the accompanyingdrawings, wherein like numbers reference like elements.

FIG. 1 is a diagram illustrating an overall configuration of ahead-mounted display of a first embodiment including an image displaydevice according to the invention.

FIG. 2 is a schematic perspective view illustrating the head-mounteddisplay illustrated in FIG. 1.

FIG. 3 is a diagram schematically illustrating the configuration of theimage display device illustrated in FIG. 1.

FIG. 4 is a diagram schematically illustrating the configuration of animage generation unit illustrated in FIG. 2.

FIGS. 5A and 5B are diagrams illustrating examples of driving signals ofa driving signal generation unit illustrated in FIG. 4.

FIG. 6 is a plan view illustrating a light scanning unit illustrated inFIG. 4.

FIG. 7 is a sectional view (a section view taken along an X1 axis)illustrating the light scanning unit illustrated in FIG. 6.

FIGS. 8A, 8B, 8C, and 8D are respectively a front view, a plan view, aright side view, and a left side view illustrating an overallconfiguration of a pupil expansion optical system illustrated in FIG. 3.

FIG. 9 is a diagram illustrating a path of video light incident on thepupil expansion optical system illustrated in FIGS. 8A to 8D.

FIG. 10 is a diagram illustrating an example of a form of the videolight when the video light scanned by the light scanning unit isprojected to a reflection unit and is scanned two-dimensionally.

FIG. 11 is a diagram illustrating an operation of the image displaydevice illustrated in FIG. 3.

FIG. 12 is a diagram illustrating an example in which the video lightscanned by the light scanning unit is projected to the reflection unitand is formed on a retina and a stray light occurrence principle.

FIG. 13 is a diagram illustrating the example in which the video lightscanned by the light scanning unit is projected to the reflection unitand is formed on the retina and the stray light occurrence principle.

FIG. 14 is a diagram illustrating the example in which the video lightscanned by the light scanning unit is projected to the reflection unitand is formed on the retina and the stray light occurrence principle.

FIGS. 15A and 15B are diagrams schematically illustrating aconfiguration of a second embodiment of the image display deviceaccording to the invention.

FIG. 16 is a diagram schematically illustrating an overall configurationof a head-up display of a third embodiment including the image displaydevice according to the invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, an image display device according to the invention will bedescribed in detail according to embodiments illustrated in theaccompanying drawings.

First Embodiment

First, a first embodiment of the image display device according to theinvention will be described.

FIG. 1 is a diagram illustrating an overall configuration of ahead-mounted display of a first embodiment including an image displaydevice according to the invention. FIG. 2 is a schematic perspectiveview illustrating the head-mounted display illustrated in FIG. 1. FIG. 3is a diagram schematically illustrating the configuration of the imagedisplay device illustrated in FIG. 1. FIG. 4 is a diagram schematicallyillustrating the configuration of an image generation unit illustratedin FIG. 2. FIGS. 5A and 5B are diagrams illustrating examples of drivingsignals of a driving signal generation unit illustrated in FIG. 4. FIG.6 is a plan view illustrating a light scanning unit illustrated in FIG.4. FIG. 7 is a sectional view (a section view taken along an X1 axis)illustrating the light scanning unit illustrated in FIG. 6. FIGS. 8A,8B, 8C and 8D are a front view, a plan view, a right side view, and aleft side view illustrating an overall configuration of a pupilexpansion optical system illustrated in FIG. 3. FIG. 9 is a diagramillustrating a path of video light incident on the pupil expansionoptical system illustrated in FIGS. 8A to 8D.

In FIGS. 1 to 3, to facilitate the description, the X, Y, and Z axes areillustrated as 3 axes that are mutually orthogonal to each other. Theleading sides and the base sides of arrows illustrated in the drawingsare referred to as “+ (positive)” and “− (negative)”, respectively. Adirection parallel to the X axis is referred to as an “X axisdirection”, a direction parallel to the Y axis is referred to as a “Yaxis direction”, and a direction parallel to the Z axis is referred toas a “Z axis direction”.

Here, the X, Y, and Z axes are set so that the X axis direction is theright and left directions of a head H, the Y axis direction is the upperand lower directions of the head H, and the Z axis direction is thefront and rear direction of the head H when the image display device 1is mounted on the head H of an observer.

As illustrated in FIG. 1, a head-mounted display (head-mounted imagedisplay device) 10 including the image display device 1 according to theembodiment has the same outer appearance as glasses, is mounted on thehead H of the observer for use, and causes the observer to view an imageformed as a virtual image so that the image overlaps with an outsideimage.

As illustrated in FIGS. 1 and 2, the head-mounted display 10 includes aframe 2 and the image display device 1 including an image generationunit 3, an expansion optical system 4, and a reflection unit 6.

In the head-mounted display 10, the image generation unit 3 generatesvideo light modulated based on a video signal, the expansion opticalsystem 4 expands a light flux width (cross-sectional area) of the videolight, and the reflection unit 6 guides the video light expanded by theexpansion optical system 4 to an eye EY of the observer. Accordingly, avirtual image according to the video signal can be caused to be viewedby the observer.

In the head-mounted display 10, the image generation units 3, theexpansion optical systems 4, and the reflection units 6 included in theimage display devices 1 are provided on the right and left sides of theframe 2 and are disposed to be bilaterally symmetric using a YZ plane asa reference. The image generation unit 3, the expansion optical system4, and the reflection unit 6 provided on the right side of the frame 2form a right-eye virtual image. The image generation unit 3, theexpansion optical system 4, and the reflection unit 6 provided on theleft side of the frame 2 form a left-eye virtual image.

In the embodiment, the head-mounted display 10 is configured to includethe image generation units 3, the expansion optical systems 4, and thereflection units 6 on the right and left sides of the frame 2 and formthe right-eye virtual image and the left-eye virtual image, but theinvention is not limited thereto. For example, the image generation unit3, the expansion optical system 4, and the reflection unit 6 may beprovided only on the left side of the frame 2 to form only the left-eyevirtual image. In contrast, the image generation unit 3, the expansionoptical system 4, and the reflection unit 6 may be provided only on theright side of the frame 2 to form only the right-eye virtual image. Thatis, the head-mounted display 10 is not limited to the binocular typehead-mounted display 10 as in the embodiment, but a monocular typehead-mounted display may be used.

Hereinafter, the units of the head-mounted display 10 will besequentially described in detail.

The two image generation units 3, the two expansion optical systems 4,and the two reflection units 6 have the same configuration. Therefore,the image generation unit 3, the expansion optical system 4, and thereflection unit 6 provided on the left side of the frame 2 will befocused on in the description.

Frame

As illustrated in FIG. 2, the frame 2 has the same shape as a glassesframe and has a function of holding the image generation unit 3, theexpansion optical system 4, and the reflection unit 6 included in theimage display device 1.

The frame 2 includes a front unit 21 that includes a rim 211 and a shadeportion 212 and temples 22 that extend in the Z axis direction from bothof the right and left ends of the front unit 21.

The shade portion 212 is a member that has a function of suppressingtransmission of outside light and holds the reflection unit 6. The shadeportion 212 has a concave portion 27 opening toward the side of theobserver therein. The reflection unit 6 is provided in the concaveportion 27. The shade portion 212 holding the reflection unit 6 is heldby the rim 211.

A nose pad 23 is provided in the middle portion of the shade portion212. The nose pad 23 comes into contact with a nose NS of the observerand supports the head-mounted display 10 with respect to the head H ofthe observer when the head-mounted display 10 is mounted on the head Hof the observer.

The temple 22 is a straight temple with no angle for putting on an earEA of the observer and a part of the temple 22 is configured to comeinto contact with the ear EA of the observer when the head-mounteddisplay 10 is mounted on the head H of the observer. The imagegeneration unit 3 and the expansion optical system 4 are accommodatedinside the temple 22.

A material for forming the temple 22 is not particularly limited. Forexample, any of various resin materials, a composite material in which acarbon fiber, a glass fiber, or the like is mixed in a resin, or a metalmaterial such as aluminum or magnesium can be used.

The shape of the frame 2 is not limited to the shape as long as theframe 2 can be mounted on the head H of the observer.

Image Display Device

As described above, the image display device 1 includes the imagegeneration unit 3, the expansion optical system 4, and the reflectionunit 6.

Hereinafter, the units of the image display device 1 according to theembodiment will be described in detail.

Image Generation Unit

As illustrated in FIG. 2, the image generation unit 3 is built in thetemple 22 of the frame 2 described above.

As illustrated in FIGS. 3 and 4, the image generation unit 3 includes avideo light generation unit 31, a driving signal generation unit 32, acontrol unit 33, a lens 34, a light diffraction unit 35, and a lightscanning unit 36.

The image generation unit 3 has a function of generating a video lightmodulated based on a video signal and a function of generating a drivingsignal to drive the light scanning unit 36.

Hereafter, the units of the image generation unit 3 will be described indetail.

Video Light Generation Unit

The video light generation unit 31 generates video light L1 to bescanned (subjected to light-scanning) by the light scanning unit 36(light scanner).

The video light generation unit 31 includes a light source unit 311including a plurality of light sources (light source units) 311R, 311G,and 311B with different wavelengths, a plurality of driving circuits312R, 312G, and 312B, and a light combination unit (combination unit)313.

The light source 311R (R light source) included in the light source unit311 emits red light, the light source 311G (G light source) emits greenlight, and the light source 311B emits blue light. A full-color imagecan be displayed using such three pieces of color light.

The light sources 311R, 311G, and 311B are not particularly limited. Forexample, laser diodes or LEDs can be used.

The light sources 311R, 311G, and 311B are electrically connected to thedriving circuits 312R, 312G, and 312B, respectively.

The driving circuit 312R has a function of driving the above-describedlight source 311R, the driving circuit 312G has a function of drivingthe above-described light source 311G, and the driving circuit 312B hasa function of driving the above-described light source 311B.

The three pieces (three colors) of light (video light) emitted from thelight sources 311R, 311G, and 311B driven by the driving circuits 312R,312G, and 312B are incident on the light combination unit 313.

The light combination unit 313 combines the pieces of light from theplurality of light sources 311R, 311G, and 311B.

In the embodiment, the light combination unit 313 includes two dichroicmirrors 313 a and 313 b.

The dichroic mirror 313 a has a function of transmitting the red lightand reflecting the green light. The dichroic mirror 313 b has a functionof transmitting the red light and the green light and reflecting theblue light.

By using the dichroic mirrors 313 a and 313 b, the three pieces oflight, the red light, the green light, and the blue light from the lightsources 311R, 311G, and 311B are combined to form one piece of videolight L1.

Here, in the embodiment, the above-described light source unit 311 isdisposed so that the light path lengths of the red light, the greenlight, and the blue light from the light sources 311R, 311G, and 311Bare mutually the same.

The light combination unit 313 is not limited to the configuration inwhich the above-described dichroic mirrors are used. For example, thelight combination unit 313 may be configured by a prism, a light-guidingpath, an optical fiber, or the like.

In the video light generation unit 31 with the above-describedconfiguration, the three color pieces of video light are generated inthe light source unit 311 and the pieces of video light are combined inthe light combination unit 313 so that one piece of video light L1 isgenerated. The video light L1 generated in the video light generationunit 31 is emitted toward the lens 34.

The above-described video light generation unit 31 may include, forexample, a light detection unit (not illustrated) that detects theintensity or the like of the video light L1 generated by the lightsources 311R, 311G, and 311B. By providing the light detection unit, itis possible to adjust the intensity of the video light L1 according to adetection result.

Lens

The video light L1 generated by the video light generation unit 31 isincident on the lens 34.

The lens 34 has a function of controlling a radiation angle of the videolight L1. The lens 34 is, for example, a collimator lens. The collimatorlens is a lens that adjusts (modulates) light to a light flux of aparallel state.

In the lens 34, the video light L1 emitted from the video lightgeneration unit 31 is transmitted in the parallel state to the lightdiffraction unit 35 (light scanning unit 36) to be described below.

Driving Signal Generation Unit

The driving signal generation unit 32 generates a driving signal todrive the light scanning unit 36 (light scanner).

The driving signal generation unit 32 includes a driving circuit 321that generates a first driving signal used for main scanning (horizontalscanning) in a first direction of the light scanning unit 36 and adriving circuit 322 that generates a second driving signal used forsub-scanning (vertical scanning) in a second direction orthogonal to thefirst direction of the light scanning unit 36.

For example, the driving circuit 321 generates the first driving signalV1 (horizontal scanning voltage) that periodically varies at a periodT1, as illustrated in FIG. 5A. The driving circuit 322 generates thesecond driving signal V2 (vertical scanning voltage) that periodicallyvaries at a period T2 different from the period T1, as illustrated inFIG. 5B.

The first and second driving signals will be described below in detailalong with description of the light scanning unit 36 to be describedbelow.

The driving signal generation unit 32 is electrically connected to thelight scanning unit 36 via a signal line (not illustrated). Accordingly,the driving signals (the first and second driving signals) generated bythe driving signal generation unit 32 are input to the light scanningunit 36.

Control Unit

The driving circuits 312R, 312G, and 312B of the video light generationunit 31 and the driving circuits 321 and 322 of the driving signalgeneration unit 32, as described above, are electrically connected tothe control unit 33. The control unit 33 has a function of controllingthe driving of the driving circuits 312R, 312G, and 312B of the videolight generation unit 31 and the driving circuits 321 and 322 of thedriving signal generation unit 32 based on video signals (imagesignals).

Based on instructions of the control unit 33, the video light generationunit 31 generates the video light L1 modulated according to imageinformation and the driving signal generation unit 32 generates adriving signal according to the image information.

Light Diffraction Unit

The video light L1 parallelized in the lens 34 is incident on the lightdiffraction unit (first diffraction optical element) 35.

The light diffraction unit 35 includes a diffraction optical elementthat diffracts the video light L1. The light diffraction unit 35configures a light reflection unit 114 of the light scanning unit 36.Therefore, a diffraction optical element included in the lightdiffraction unit 35 functions as a reflective diffraction element.Accordingly, the video light L1 incident on the light diffraction unit35 is reflected by the light diffraction unit 35 (light reflection unit114) and mutually intensifies light at a specific diffraction angledecided for each wavelength. Accordingly, diffracted light with arelatively large intensity at the specific angle is generated.

In the embodiment, the light diffraction unit 35 is configured by afirst hologram element 351 which is one diffraction grating. The firsthologram element 351 is a semi-transmissive film that has propertiesdiffracting light in a specific wavelength region and transmitting lightin the other wavelength region in the video light L1 incident on thelight diffraction unit 35.

By using the first hologram element 351 in the light reflection unit 114of the light scanning unit 36 to be described below, it is possible todiffract and reflect the video light L1 in a specific wavelength bandand guides the video light L1 to the expansion optical system 4.

As the diffraction grating included in the light diffraction unit 35,any diffraction grating may be used as long as the diffraction gratingcan be provided to overlap with a plate-shaped member 113 and function areflective diffraction element. Besides the above-described hologramelement (holographic grating), a surface release type diffractiongrating (blazed grating) in which a groove having a cross section with asawtooth shape is formed or a surface relief hologram element (blazedholographic grating) in which a hologram element and a surface relieftype diffraction grating are combined may be used.

Of these elements, a surface blazed hologram element is preferably usedwhen diffraction efficiency is considered to be important. This elementcan obtain particularly high diffraction efficiency by matching thewavelength (the wavelength of light with the highest diffractionefficiency) of diffracted light decided by an angle (blazed angle) of asurface forming a groove, the wavelength of diffracted light decided byan interference fringe pitch of a hologram element, and the wavelengthof the video light L1.

The function of the light diffraction unit 35 will be described below indetail.

Light Scanning Unit

The video light L1 emitted from the video light generation unit 31 isincident on the light diffraction unit 35 (the light scanning unit 36)via the lens 34.

The light scanning unit 36 is a light scanner that two-dimensionallyscans the video light L1 from the video light generation unit 31. Ascanning light (video light) L2 is formed when the light scanning unit36 scans the video light L1.

As illustrated in FIG. 6, the light scanning unit 36 includes a movablemirror 11, one pair of axis portions 12 a and 12 b (first axis portion),a frame body 13, two pairs of axis portions 14 a, 14 b, 14 c, and 14 d(second axis portion), and a support portion 15, and a permanent magnet16, and a coil 17. In other words, the light scanning unit 36 has aso-called gimbal structure.

Here, the movable mirror 11 and the one pair of axis portions 12 a and12 b configure a first vibration system that sways (reciprocates androtates) around a Y1 axis (first axis). The movable mirror 11, the onepair of axis portions 12 a and 12 b, the frame body 13, the two pairs ofaxis portions 14 a, 14 b, 14 c, and 14 d, and the permanent magnet 16configure a second vibration system that sways (reciprocates androtates) around an X1 axis (second axis).

The light scanning unit 36 includes a signal superimposition unit 18(see FIG. 7). The permanent magnet 16, the coil 17, the signalsuperimposition unit 18, and the driving signal generation unit 32configure a driving unit that drives the above-described first andsecond vibration systems (that is, sways the movable mirror 11 aroundthe X1 axis and the Y1 axis).

Hereinafter, the units of the light scanning unit 36 will besequentially described in detail.

The movable mirror 11 includes a base portion 111 (movable portion) anda plate-shaped member 113 fixed to the base portion 111 via a spacer112.

The light diffraction unit 35 described above as the light reflectionunit 114 is provided on the upper surface (one surface) of theplate-shaped member 113.

As illustrated in FIG. 7, a light absorption unit 116 absorbing incidentlight may be provided between the light diffraction unit 35 and theplate-shaped member 113. In this case, the light absorption unit 116absorbs light which is incident on the light diffraction unit 35 and maynot be diffracted to prevent the light from becoming stray light.

In the embodiment, the plate-shaped member 113 is formed in a circularshape in a plan view. The shape of the plate-shaped member 113 in theplan view is not limited thereto. For example, a circular shape such asan elliptical shape or an oval shape, a tetragonal shape, or a polygonalshape such as a hexagonal shape may be used.

A hard layer 115 is provided on the lower surface (the other surface) ofthe plate-shaped member 113, as illustrated in FIG. 7.

The hard layer 115 is formed of a harder material than a material of thebody of the plate-shaped member 113. Accordingly, it is possible toimprove the rigidity of the plate-shaped member 113. Therefore, it ispossible to prevent or suppress bending at the time of swaying of theplate-shaped member 113. By thinning the thickness of the plate-shapedmember 113, it is possible to prevent the moment of inertia when theplate-shaped member 113 is swayed around the X1 axis and the Y1 axis.

The material of the hard layer 115 is not particularly limited as longas the material is a material harder than the material of the body ofthe plate-shaped member 113. For example, diamond, a carbon nitridefilm, crystal, sapphire, lithium tantalate, or potassium niobate can beused.

The hard layer 115 may be configured by a single layer or may be alaminate of a plurality of layers. The hard layer 115 is provided asnecessary, and thus can be omitted.

The lower surface of the plate-shaped member 113 is fixed to the baseportion 111 via the spacer 112. Accordingly, it is possible to sway theplate-shaped member 113 around the Y1 axis while preventing contactbetween the plate-shaped member 113, and the axis portions 12 a and 12b, the frame body 13, the axis portions 14 a, 14 b, 14 c, and 14 d.

As illustrated in FIG. 6, the frame body 13 is formed in a frame shapeand is provided to surround the base portion 111 of the movable mirror11 described above. In other words, the base portion 111 of the movablemirror 11 is provided inside the frame body 13 formed in the frameshape.

The frame body 13 is supported by the support portion 15 via the axisportions 14 a, 14 b, 14 c, and 14 d. The base portion 111 of the movablemirror 11 is supported by the frame body 13 via the axis portions 12 aand 12 b.

The axis portions 12 a and 12 b connect the movable mirror 11 to theframe body 13 so that the movable mirror 11 can be rotated (sway) aroundthe Y1 axis. The axis portions 14 a, 14 b, 14 c, and 14 d connect theframe body 13 to the support portion 15 so that the frame body 13 can berotated (sway) around the X1 axis orthogonal to the Y1 axis.

The axis portions 12 a and 12 b are disposed to face each other via thebase portion 111 of the movable mirror 11. The axis portions 12 a and 12b form a longitudinal shape extending in the direction along the Y1axis. One end of each of the axis portions 12 a and 12 b is connected tothe base portion 111 and the other end thereof is connected to the framebody 13. The axis portions 12 a and 12 b are disposed so that eachcentral axis matches the Y1 axis.

The axis portions 12 a and 12 b are twisted and deformed with theswaying of the movable mirror 11 around the Y1 axis.

The axis portions 14 a and 14 b and the axis portions 14 c and 14 d aredisposed to face each other via (interleaving) the frame body 13. Theaxis portions 14 a, 14 b, 14 c, and 14 d form a longitudinal shapeextending in the direction along the X1 axis. One end of each of theaxis portions 14 a, 14 b, 14 c, and 14 d is connected to the frame body13 and the other end thereof is connected to the support portion 15. Theaxis portions 14 a and 14 b are disposed to face each other via the X1axis. Similarly, the axis portions 14 c and 14 d are disposed to faceeach other via the X1 axis.

For the axis portions 14 a, 14 b, 14 c, and 14 d, the entire axisportions 14 a and 14 b and the entire axis portions 14 c and 14 d aretwisted and deformed with the swaying of the frame body 13 around the X1axis.

In this way, by enabling the movable mirror 11 to be swayed around theY1 axis and enabling the frame body 13 to be swayed around the X1 axis,it is possible to sway (reciprocate and rotate) the movable mirror 11around the two axes, the X1 axis and the Y1 axis orthogonal to eachother.

Although not illustrated, for example, an angle detection sensor such asa strain sensor is provided in at least one of the axis portions 12 aand 12 b and at least one of the axis portions 14 a, 14 b, 14 c, and 14d. The angle detection sensor can detect angle information regarding thelight scanning unit 36 and, more specifically, each swaying angle of thelight reflection unit 114 around the X1 axis and the Y1 axis. Thedetection result is input to the control unit 33 via a cable (notillustrated).

The permanent magnet 16 is joined to the lower surface (the oppositesurface to the plate-shaped member 113) of the above-described framebody 13.

In the embodiment, the permanent magnet 16 has a longitudinal shape (barshape) and is disposed in a direction inclined to the X1 axis and the Y1axis. The permanent magnet 16 is magnetized in the longitudinaldirection. That is, the permanent magnet 16 is magnetized such that oneend of the permanent magnet 16 serves as the S pole and the other endthereof serves as the N pole.

In the embodiment, the case in which one permanent magnet is provided inthe frame body 13 has been exemplified, but the invention is not limitedthereto. For example, two permanent magnets may be provided in the framebody 13. In this case, for example, two permanent magnets formed in along shape may be provided in the frame body 13 so that the permanentmagnets face each other and are parallel to each other via the baseportion 111 in a plan view.

The coil 17 is provided immediately below the permanent magnet 16. Thatis, the coil 17 is provided to face the lower surface of the frame body13. Accordingly, it is possible to operate a magnetic field generatedfrom the coil 17 to the permanent magnet 16, and it is possible torotate the movable mirror 11 around each of the two axes (the X1 axisand the Y1 axis) orthogonal to each other.

The coil 17 is electrically connected to the signal superimposition unit18 (see FIG. 7).

When the signal superimposition unit 18 applies a voltage to the coil17, a magnetic field with a magnetic flux orthogonal to the X1 axis andthe Y1 axis is generated from the coil 17.

The signal superimposition unit 18 includes an adder (not illustrated)that superimposes the first driving signal V1 and the second drivingsignal V2 described above and applies the superimposed voltage to thecoil 17.

The driving circuit 321 generates, for example, the first driving signalV1 (horizontal scanning voltage) that periodically varies at the periodT1, as illustrated in FIG. 5A. That is, the driving circuit 321generates the first driving signal V1 with a first frequency (1/T1).

The first driving signal V1 forms a waveform such as a sinusoidal wave.Therefore, the light scanning unit 36 can efficiently perform mainscanning on the light. The waveform of the first driving signal V1 isnot limited thereto.

The first frequency (1/T1) is not particularly limited as long as thefirst frequency is a frequency proper for horizontal scanning and ispreferably 10 kHz to 40 kHz.

In the embodiment, the first frequency is set to be the same as atorsional resonant frequency (f1) of the first vibration system(torsional vibration system) configured to include the movable mirror 11and the one pair of axis portions 12 a and 12 b. That is, the firstvibration system is designed (manufactured) so that the torsionalresonant frequency f1 is a frequency proper for the horizontal scanning.Accordingly, a rotational angle of the movable mirror 11 around the Y1axis can be enlarged.

On the other hand, the driving circuit 322 generates, for example, thesecond driving signal V2 (vertical scanning voltage) that periodicallyvaries at the period T2 different from the period T1, as illustrated inFIG. 5B. That is, the driving circuit 322 generates the second drivingsignal V2 with a second frequency (1/T2).

The second driving signal V2 forms a waveform such as a sawtooth wave.Therefore, the light scanning unit 36 can efficiently perform verticalscanning (sub-scanning) on the light. The waveform of the second drivingsignal V2 is not limited thereto.

In the embodiment, the frequency of the second driving signal V2 isadjusted so that the frequency is a different frequency from a torsionalresonant frequency (resonant frequency) of the second vibration system(torsional vibration system) configured to include the movable mirror11, the one pair of axis portions 12 a and 12 b, the frame body 13, thetwo pairs of axis portions 14 a, 14 b, 14 c, and 14 d, and the permanentmagnet 16.

In a raster scanning scheme which is a video drawing scheme, theabove-described vertical scanning is performed while performing theabove-described horizontal scanning. At this time, the frequency of thehorizontal scanning is set to be higher than the frequency of thevertical scanning. In general, in the raster scan scheme, scanning at ahigh frequency is referred to as main scanning and scanning at a lowfrequency is referred to as sub-scanning.

In the above-described light scanning unit 36, the movable mirror 11including the light reflection unit 114 is swayed around each of the twoaxes orthogonal to each other, and thus the light scanning unit 36 canbe miniaturized and become lightweight. As a result, the observer canmore easily use the image display device 1.

In particular, since the light scanning unit 36 has the gimbalstructure, it is possible to miniaturize the configuration (the lightscanning unit 36) that scans the video light two-dimensionally.

Expansion Optical System

As illustrated in FIG. 3, the scanning light (video light) L2 scanned bythe above-described light scanning unit 36 is transmitted to theexpansion optical system 4.

The expansion optical system 4 has a function of expanding a light fluxwidth of the video light L2 scanned by the light scanning unit 36, thatis, expanding the cross-sectional area of the video light L2.

Any optical system can be used as the expansion optical system 4 as longas the optical system has such a function and the configuration is notparticularly limited. As illustrated in FIG. 3, for example, theexpansion optical system 4 according to the embodiment includes anoptical element 5, a correction lens 42, and a light-shielding plate 43.The image display device 1 according to the embodiment includes such anexpansion optical system 4, but this function may be omitted when thisfunction is not necessary.

Hereinafter, the units of the expansion optical system 4 will besequentially described in detail.

Optical Element

As illustrated in FIG. 3, the optical element 5 is provided near thelight scanning unit 36, has light transparency (light transmissiveproperty), and has a long shape along the Z axis direction.

The video light L2 scanned by the above-described light scanning unit 36is incident on the optical element 5.

The optical element 5 expands the light flux width (cross-sectionalarea) of the video light L2 scanned by the light scanning unit 36.Specifically, the optical element 5 expands the light flux width of thevideo light L2 by propagating the video light L2 scanned by the lightscanning unit 36 in the Z direction while multiply reflecting the videolight L2 inside the optical element 5 and emits pieces of video light L3and L4 with the larger light flux width than the video light L2. Such anoptical element 5 functions as a pupil expansion optical system.

As illustrated in FIGS. 8A to 8D, the optical element 5 has an incidentsurface 56 at one end in the longitudinal direction (the Z axisdirection) and an emission surface 57 at the other end. These surfaces(the incident surface 56 and the emission surface 57) face each other.The optical element 5 has side surfaces 58 a and 58 b facing each otherin the thickness direction (the X axis direction) and side surfaces 59 aand 59 b facing in the width direction (the Y axis direction).

The incident surface 56 is provided to confront the light scanning unit36 and the emission surface 57 is provided to confront the correctionlens 42 and the light-shielding plate 43 (see FIG. 3).

The incident surface 56 is a surface that has light transparency and isa surface on which the video light L2 scanned by the light scanning unit36 is incident. On the other hand, the emission surface 57 is a surfacethat has light transparency and is a surface from which the video lightL2 incident from the incident surface 56 is emitted as the pieces ofvideo light L3 and L4.

The side surfaces 58 a and 58 b are total reflection surfaces andtotally reflect the video light L2 incident inside the optical element5. Here, the total reflection surfaces include not only surfaces withlight transmittance of 0% but also surfaces that slightly transmitlight, for example, surfaces with light transmittance less than 3%.

The side surfaces 59 a and 59 b may be surfaces with any lighttransmittance. For example, the side surfaces 59 a and 59 b may be totalreflection surfaces or semi-reflection surfaces, but are preferablysurfaces with relatively low light transmittance. Accordingly, it ispossible to prevent the light inside the optical element 5 from becomingstray light. As a method of preventing the light inside the opticalelement 5 from becoming stray light, for example, a method of rougheningthe side surfaces 59 a and 59 b.

As illustrated in FIGS. 8A to 8D, the incident surface 56 and theemission surface 57 are parallel to each other. The side surfaces 58 aand 58 b are parallel to each other. The side surfaces 59 a and 59 b areparallel to each other. Therefore, in the embodiment, the entire shapeof the optical element 5 is rectangular parallelepiped.

The foregoing “parallelism” includes not only complete parallelism butalso parallelism in which an angle formed by the surfaces is ±2°.

In the embodiment, the incident surface 56 and the emission surface 57are parallel to each other. However, the incident surface 56 and theemission surface 57 may not be parallel to each other and the absolutevalues of inclination angles may be the same. The fact that “theabsolute values of the inclination angles of the incident surface 56 andthe emission surface 57 are the same” includes, for example, a state inwhich the incident surface 56 is inclined at an acute angle α (forexample, +20°) in the +Z axis direction with respect to the XY plane andthe emission surface 57 is inclined at the acute angle α(for example,−20°) in the −Z axis direction with respect to the XY plane.

In the embodiment, the side surfaces 59 a and 59 b are parallel to eachother. However, the side surfaces 59 a and 59 b may not be parallel toeach other or inclination angles may be different.

As illustrated in FIGS. 8A to 8D, the optical element 5 having such aconfiguration includes light-guiding units 51, 52, and 53 that guide thevideo light L2 and half mirror layers 54 and 55.

The optical element 5 is configured such that the light-guiding unit 51,the half mirror layer 54, the light-guiding unit 52, the half mirrorlayer 55, and the light-guiding unit 53 are stacked in this order in thethickness direction (the X axis direction). That is, the optical element5 is a one-dimensional array in which the light-guiding units 51, 52,and 53 are arrayed in the thickness direction with the half mirrorlayers 54 and 55 interposed therebetween.

The light-guiding units 51, 52, and 53 are light pipes formed in a plateshape and have a function of propagating the video light L2 (the videolight scanned by the light scanning unit 36) incident from the incidentsurface 56 in the +Z direction.

As illustrated in FIGS. 8A and 8B, the cross-sectional shapes (thecross-sectional shapes on the XY plane) of the light-guiding units 51,52, and 53 have a rectangular shape. The cross-sectional shapes (thecross-sectional shapes on the XY plane) of the light-guiding units 51,52, and 53 are not limited thereto, but may have a tetragonal shape suchas a square shape or another polygonal shape.

The light-guiding units 51, 52, and 53 may have light transparency andare formed of, for example, any of various resin materials such as anacrylic resin or a poly carbonate resin or any of various kinds ofglass.

The half mirror layers 54 and 55 are formed of, for example, areflection film having light transparency, that is, a semi-transflectivefilm. The half mirror layers 54 and 55 have a function of reflecting apart of the video light L2 and transmitting a part of the video lightL2. The half mirror layers 54 and 55 are formed of a semi-transflectivefilm such as a metal reflection film of sliver (Ag), aluminum (Al), orthe like or a dielectric multi-layer film.

The optical element 5 having such a configuration can be obtained, forexample, by performing surfactant bonding on the light-guiding units 51,52, and 53 in which thin films serving as the half mirror layers 54 and55 are formed on the main surfaces. By manufacturing the optical element5 by the surfactant bonding, it is possible to improve the degree ofparallelism of the units (the light-guiding units 51, 52, and 53).

In the optical element 5 having the above-described configuration, asillustrated in FIG. 9, the video light L2 scanned by the light scanningunit 36 is incident from the incident surface 56, is multiply reflectedinside the optical element 5, and is emitted as the pieces of videolight L3 and L4 in the state of the expanded light flex width from theemission surface 57. In this way, the light flux width (thecross-sectional area) of the video light L2 can be expanded in theoptical element 5.

Here, by parallelizing the incident surface 56 and the emission surface57, the amount of refraction of the video light L2 incident on theincident surface 56 can be the same as the amount of refraction of thepieces of video light L3 and L4 emitted from the emission surface 57.That is, an angle θ5 at which the video light L2 is incident withrespect to the half mirror layers 54 and 55 can be the same as angles θ5at which the pieces of video light L3 and L4 are emitted with respect tothe half mirror layers 54 and 55. Accordingly, it is possible to preventdistortion caused by a trigonometric function of the rule of refractionor occurrence of chromatic aberration caused by wavelength dispersion ofthe refractive index of the material.

The optical element 5 according to the embodiment is a one-dimensionalarray (first one-dimensional array) in which the light-guiding units 51,52, and 53 are arrayed in the thickness direction. In this way, in therelatively simple configuration in which the light-guiding units 51, 52,and 53 are mutually stacked, the video light L2 incident from theincident surface 56 is multiply reflected inside the optical element 5.Therefore, without using a position detection unit or the like matchingthe video light with a visual line of the observer or the positions ofthe right and left eyes EY of the observer, the light flux width of thevideo light L2 can be expanded in the relatively simple configurationaccording to the embodiment.

As illustrated in FIG. 3, the optical element 5 is disposed so thatprincipal rays of the pieces of video light L3 and L4 are emitted fromthe reflection unit 6 in an in-plane direction (XY in-plane direction)including an axis line W (see FIG. 1) parallel to a direction (the Xaxis direction) in which the left eye EY and the right eye EY of theobserver are arranged when the head-mounted display is mounted on thehead H of the observer. In other words, the optical element 5 isdisposed so that the cross-sectional area of the video light L3 isexpanded in the direction of the axis line W. The correction lens 42 andthe light-shielding plate 43 are arranged along the axis line W.Therefore, the video light L3 emitted from the emission surface 57 isemitted toward the reflection unit 6 via the correction lens 42 and thevideo light L4 emitted from the emission surface 57 is emitted towardthe light-shielding plate 43. In this way, by disposing the opticalelement 5 so that the cross-sectional area of the video light L3 in thedirection of the axis line W is expanded, it is possible to expand thevideo light L3 guided to the eye of the observer via the correction lens42 and the reflection unit 6 in the right and left directions of theeye. Accordingly, it is possible to improve visibility in the right andleft directions in which a movement range is larger than the upper andlower directions of the eye.

Correction Lens

As illustrated in FIG. 3, the video light L3 emitted from the opticalelement 5 is incident on the correction lens 42.

The correction lens 42 has a function of correcting disturbance of theparallelism of the video light L3 by an aspheric mirror 61 included inthe reflection unit 6 to be described below. Accordingly, it is possibleto improve the resolution performance of the video light L3. Examples ofthe correction lens 42 include a toroidal lens, a cylindrical lens, anda free curved lens.

Light-Shielding Plate

The video light L4 emitted from the optical element 5 is incident on thelight-shielding plate 43.

The light-shielding plate 43 is configured to include a light absorptionmember that absorbs light and is a light-shielding unit that shieldslight. Accordingly, the video light L4 emitted from the optical element5 is shielded as unnecessary light.

Such a light-shielding plate 43 is formed of, for example, a stainlesssteel or an aluminum alloy.

In the embodiment, the light-shielding plate 43 is used as thelight-shielding unit that shields the video light L4. However, thelight-shielding unit that shields the video light L4 is not limitedthereto. The video light L4 may be prevented from becoming stray light.For example, the light-shielding plate 43 may not be used as thelight-shielding unit, but the light-shielding unit may have aconfiguration in which the video light L4 is shielded by applying apaint or the like to the circumference of the frame 2.

The video light L3 of which the light flux width is expanded by theexpansion optical system 4 with the above-described configuration isincident on the reflection unit 6 via the correction lens 42, asillustrated in FIG. 3.

Reflection Unit

The reflection unit 6 is provided in the shade portion 212 of the frontunit 21 and is disposed to be located in front of the left eye EY of theobserver at the time of use. The reflection unit 6 has a sufficient sizeto cover the eye EY of the observer and has a function of causing thevideo light L3 from the optical element 5 to be incident toward the eyeEY of the observer.

The reflection unit 6 includes the aspheric mirror 61 including a lightdiffraction unit (the second diffraction optical element) 65.

The aspheric mirror 61 is a light transparent member in which asemi-transflective film is manufactured on a substrate formed of a resinmaterial with light transparency (light transmissive property) of a highvisible range. That is, the aspheric mirror 61 is a half mirror and hasa function of transmitting outside light (light transparency withrespect to the visible light). Accordingly, the reflection unit 6including the aspheric mirror 61 has a function of reflecting the videolight L3 emitted from the optical element 5 and transmitting the outsidelight traveling toward the eye EY of the observer from the outside ofthe reflection unit 6 at the time of use. Accordingly, the observer canview a virtual image (image) formed by the video light L5 while viewingan outside image. That is, the see-through head-mounted display can berealized.

Such an aspheric mirror 61 is formed in a shape curved along the curveof the front unit 21 of the frame 2 and a concave surface 611 is locatedon the side of the observer at the time of use. Accordingly, the videolight L5 reflected by the aspheric mirror 61 can efficiently becondensed toward the eye EY of the observer.

The light diffraction unit 65 is provided on the concave surface 611.The light diffraction unit 65 has a function of deflecting the videolight L3 emitted from the emission surface 57 of the optical element 5in the direction of the eye EY of the observer by diffraction. That is,the light diffraction unit 65 includes a diffraction optical elementthat diffracts the video light L3. Since the diffraction optical elementis a reflective diffraction element, the video light L3 incident on thelight diffraction unit 65 is reflected and the light is mutuallyintensified at a specific angle decided for each wavelength.Accordingly, the diffracted light with a relatively great intensity isgenerated at a specific diffraction angle.

In the embodiment, the light diffraction unit 65 is configured toinclude a second hologram element 651 which is one diffraction grating.The second hologram element 651 is a semi-transmissive film that hasproperties diffracting light in a specific wavelength region andtransmitting light in the other wavelength region in the video light L3radiated from the optical element 5 to the second hologram element 651.

By using such a second hologram element 651, the angle or the light fluxstate of the video light guiding to the eye of the observer can beadjusted by diffraction in the video light in the specific wavelengthband, and thus the virtual image can be formed in front of the eye.Specifically, the video light L3 reflected by the aspheric mirror 61 isemitted to the outside and is incident as the video light L5 to the lefteye EY of the observer by the second hologram element 651. The same alsoapplies to the reflection unit 6 located on the side of the right eyeEY. The video light L5 incident on each of the right and left eyes EY ofthe observer is formed as an image on the retina of the observer.Accordingly, the observer can observe the virtual image (image) formedby the video light L3 emitted from the optical element 5 in a visualfield range.

Any diffraction element may be used as the diffraction grating includedin the light diffraction unit 65 as long as the diffraction element is areflective diffraction element. Besides the above-described hologramelement (holographic grating), a surface release type diffractiongrating (blazed grating) in which a groove having a cross section with asawtooth shape is formed or a surface relief hologram element (blazedholographic grating) in which a hologram element and a surface relieftype diffraction grating are combined may be used.

In the above-described image display device 1, the video light L1generated by the image generation unit 3 is expanded by the expansionoptical system 4 and is guided to the eye EY of the observer in thereflection unit 6, so that the observer can recognize the video lightgenerated by the image generation unit 3 as a virtual image formed in avisual field range of the observer.

Operation of Image Display Device

FIG. 10 is a diagram illustrating an example of a form of the videolight when the video light scanned by the light scanning unit isprojected to a reflection unit and is scanned two-dimensionally.

In the example illustrated in FIG. 10, the video light L3 canned by thelight scanning unit 36 and expanded in the expansion optical system 4 isprojected inside the rectangular second hologram element 651 (the lightdiffraction unit 65) of the aspheric mirror 61 of the reflection unit 6.

The video light L3 draws any video inside the second hologram element651 by combining the main scanning in the horizontal direction (theright and left directions of FIG. 10) and the sub-scanning in thevertical direction (the upper and lower directions of FIG. 10). Ascanning pattern of the video light L3 is not particularly limited. In apattern example indicated by a dotted line arrow in FIG. 10, motions ofperforming the main scanning in the horizontal direction, subsequentlyperforming the sub-scanning in the vertical direction at an end andperforming the main scanning in the opposite direction to the horizontaldirection, and subsequently performing the sub-scanning in the verticaldirection at an end are repeated.

FIG. 11 is a diagram illustrating an operation of the image displaydevice illustrated in FIG. 3. In FIG. 11, the expansion optical system 4and the like are not illustrated.

A diffraction angle in the light diffraction unit 65 depends on thewavelength of the video light L3 incident on the light diffraction unit65. When the video light L3 has only completely monochromatic light,that is, light with a specific wavelength, the diffraction angle of thevideo light L3 is normally constant and the emission direction of thevideo light L5 incident on the eye EY of the observer is also normallyconstant. Therefore, the observer can view a clear image with no blur orsmear without deviation in the position of the virtual image recognizedby the observer.

However, it is not easy to cause the video light L3 to have thecompletely monochromatic light, in other words, to cause the video lightL1 incident on the light diffraction unit 35 in FIG. 3 to have thecompletely monochromatic light. Further, a wavelength width of, forexample, a few of nm is contained although the wavelength width differsdepending on a kind of the light source unit 311. In particular, thistendency is prominent when a semiconductor laser of a verticalmulti-mode is used as a light source. In a configuration of the relatedart in which the light diffraction unit 35 is not included, the videolight L1 with such a wavelength width is incident on the lightdiffraction unit 65 and is diffracted, and a predetermined angle widthoccurs even at a diffraction angle according to a wavelength width of,for example, a few of nm. As a result, the video light L5 has this anglewidth and is incident as the video light L5 on the eye EY of theobserver. Since an angle deviation affects a position deviation in theretina of the observer more considerably than deviation in the positionat which the video light is incident on the eye EY of the observer, alarge position deviation corresponding to several pixels to several tensof pixels occurs in the retina of the observer in the configuration ofthe related art.

As an example of a calculation result of the position deviation, whenthe light incident on the light diffraction unit 65 is green light andthe wavelength of the green light deviates by 1 nm (when a wavelengthwidth occurs), a position deviation corresponding to 3.4 pixels on theretina accordingly occurs. When the light incident on the lightdiffraction unit 65 is blue light and the wavelength of the blue lightdeviates by 1 nm (when a wavelength width occurs), a position deviationcorresponding to 3.9 pixels on the retina accordingly occurs. When thelight incident on the light diffraction unit 65 is red light and thewavelength of the red light deviates by 1 nm (when a wavelength widthoccurs), a position deviation corresponding to 2.7 pixels on the retinaaccordingly occurs. The position deviation of such a virtual imageresults in a deterioration in the resolution of a video recognized bythe observer. In other words, the image quality of the video decreases.

When the temperature of the light source unit 311 changes with a changein environment temperature, the wavelength of the output light changesin accordance with the temperature characteristics of the light sourceunit 311. Accordingly, when the wavelength of the video light L3changes, the diffraction angle changes in the light diffraction unit 65and a position at which the video light L5 is formed as an image may beaccordingly deviated. At this time, when the temperature characteristicsare mutually the same in the light source 311R emitting the red light,the light source 311G emitting the green light, and the light source311B emitting the blue light, the deviations in the positions of thethree pieces of color light formed as the image are also the same.Therefore, movement (shift) of a video occurs, but a color deviationdoes not occur.

However, the temperature characteristics are generally different in thelight sources 311R, 311G, and 311B. In this case, when the environmenttemperature is changed, a difference in a change width of the wavelengthoccurs for each color of the light. As a result, in the configuration ofthe related art, for example, the position of the formed image differsin the video light L5 of the red, the video light L5 of the green, andthe video light L5 of the blue, and thus a so-called color deviationoccurs in addition to the shift of the video.

Further, when the outputs of the light sources 311R, 311G, and 311B arechanged to modulate the intensity of the video light L5 (directlymodulated), the wavelength of the output light changes with a change ina driving current in some cases. When the wavelength changes, thewavelength of the video light L3 changes based on an intensitymodulation signal, and thus the diffraction angle in the lightdiffraction unit 65 also changes over time based on the intensitymodulation signal. As a result, in the configuration of the related art,whenever the intensity of the video light L5 is modulated, a position atwhich the video light L5 is formed as an image may be deviated, therebyresulting in the deterioration in the resolution of the video viewed bythe observer.

To resolve such a problem, in the embodiment, the light diffraction unit35 is provided to overlap with the light reflection unit 114 of thelight scanning unit 36. When the light is incident on the lightdiffraction unit 35, an angle width is accompanied at a diffractionangle based on the wavelength width of the incident light (the videolight L1), as in the light diffraction unit 65. For example, when thewavelength width of a few of nm is present in the video light L1, thediffraction angle of the light emitted from the light diffraction unit35 is decided based on the shape of the first hologram element 351included in the light diffraction unit or the wavelength of the videolight L1, and thus a predetermined angle width corresponding to thewavelength width is accompanied. In the example illustrated in FIG. 11,the video light L1 is diffracted in the light diffraction unit 35, andthus the video light L3 and video light L3′ are formed as examples ofthe video light propagating to extend at a predetermined angle. In thefollowing description, to facilitate the description, diffraction of thevideo light L1 in the light diffraction unit 35 is referred to as “firstdiffraction”.

The video light L3 and the video light L3′ accompanying a predeterminedangle width in such first diffraction are incident on the reflectionunit 6 via the light scanning unit 36 and the expansion optical system4. Then, as described above, diffraction occurs again in the video lightL3 and the video light L3′ incident on the light diffraction unit 65provided in the reflection unit 6. In the following description, tofacilitate the description, diffraction of the video light L3 and thevideo light L3′ in the light diffraction unit 65 is referred to as“second diffraction”.

In the second diffraction, a predetermined angle width corresponding tothe wavelength width is also accompanied since the diffraction angle ofthe light emitted from the light diffraction unit 65 is decided based onthe shape of the second hologram element 651 included in the lightdiffraction unit 65 and the wavelengths of the video light L3 and thevideo light L3′.

Here, in the second diffraction, the diffraction occurs so that theangle width of the diffraction angles occurring in the first diffractionis offset (corrected). As a result, the angle widths of the diffractionangles of the video light L3 and the video light L3′ emitted from thelight diffraction unit 65 are suppressed to be small. Accordingly, it ispossible to suppress the deviation in the image formation positions ofthe video light L5 and the video light L5′ on the retina of the observerto be small. That is, when there is no second diffraction, the videolight L3 and the video light L3′ continuously extend at predeterminedangles, and thus are incident on the eye with a angle difference.Therefore, the resolution may deteriorate on the retina. However, sinceat least part of the angle width occurring in the first diffraction isoffset in the second diffraction, the angle difference between the videolight L5 which is the diffracted light of the video light L3 and thevideo light L5′ which is the diffracted light of the video light “L3’sufficiently decreases (that is, the video light L5 and the video lightL5′ approach each other in parallel), as illustrated in FIG. 11, andthus the difference in the image formation position in the retina of theobserver sufficiently decreases. As a result, it is possible to suppressthe deterioration in the resolution of the video.

Similarly, by undergoing the diffraction twice, at least part of theangle width of the diffraction angles occurring in the first diffractionis offset in the second diffraction even when the environmenttemperature changes and the wavelength of the light output from thelight source unit 311 changes. Therefore, the angle width can beconfigured not to increase further. As a result, the angle width of thediffraction angles in the second diffraction can be suppressed to besmall, and thus occurrence of the color deviation can be suppressed tobe small.

Similarly, by undergoing the diffraction twice, at least part of theangle width of the diffraction angles occurring in the first diffractioncan be offset in the second diffraction even when the light sources311R, 311G, and 311B are directly modulated. As a result, the anglewidth of the diffraction angles in the second diffraction can besuppressed to be small. Accordingly, the deviation in the position atwhich the video light L5 is formed as the image on the retina of theobserver can be suppressed to be small.

As described above, in the embodiment, even when the video light L1accompanies the wavelength width, the change width of the wavelengthdiffers for each color of the light, or the wavelength changes overtime, an increase in the angle width of the diffraction angles due tothe wavelength width, an increase in the change width of the diffractionangles over time or for each color due to the wavelength change issuppressed in the video light L5. Accordingly, the position at which thevideo light L5 is formed as the image is suppressed to, for example, thedegree equal to or less than one pixel, the deterioration in the imagequality is suppressed, and the deterioration in the image quality causeddue to the color deviation is also suppressed.

To offset the angle width occurring between the diffraction angles inthe first diffraction and the second diffraction as reliable aspossible, a grating period of the diffraction grating used in the firstdiffraction and a grating period of the diffraction grating used in thesecond diffraction may approach each other as close as possible.

In the embodiment, the first hologram element 351 is used as the lightdiffraction unit 35 carrying out the first diffraction and the secondhologram element 651 is used as the light diffraction unit 65 carryingout the second diffraction. Since diffraction occurs in the hologramelement based on an interference fringe which is a diffraction gratingrecorded on the hologram element, the first hologram element 351 and thesecond hologram element 651 may be configured such that pitches of thedistances of the interference fringes (diffraction grating periods) areas mutually close as possible. Even when a surface relief typediffraction grating is used in the first diffraction and the secondhologram element 651 is used in the second diffraction, the gratingpitch of the surface relief type diffraction grating and theinterference fringe pitch of the second hologram element 651 may beconfigured to be mutually as close as possible. In the followingdescription, the interference fringe will be mainly described, but theregulations of the interference fringe can also be applied to adiffraction grating structure such as a grating or a groove withoutchange.

The first hologram element 351 may have portions in which theinterference fringe pitches are mutually different. However, it isassumed that the interference fringe pitches are constant in the entirehologram element. Since it is easy to design and manufacture the firsthologram element 351, it is possible to obtain the advantage of easilyachieving high precision of the interference fringe pitches and realizelow cost.

In the regulation of “constant interference fringe pitch” in this case,for example, a variation in the interference fringe pitch caused in amanufacturing process is allowed.

The interference fringe pitch (the diffraction grating period) of thefirst hologram element 351 refers to a pitch obtained on a line drawn topass through a point to which the video light L1 is projected and to beorthogonal to the interference fringe in the first hologram element 351.

In contrast, the second hologram element 651 preferably has portions inwhich the interference fringe pitches are mutually different.Specifically, since diffraction angles at which the video light L3 isdiffracted to be incident on the eye EY of the observer are mutuallydifferent, for example, among a central portion 651 a, an end portion651 b on the side of the image generation unit 3, and an end portion 651c on the opposite side to the image generation unit 3 in the secondhologram element 651 illustrated in FIG. 11, it is preferable that theinterference fringe pitches are accordingly different from each other.Accordingly, it is possible to diffract the video light L3 projected tothe second hologram element 651 while scanned two-dimensionally so thatthe video light L3 is incident on the eye EY of the observer. As aresult, the observer can view a video with a large angle of view andhigh quality.

As an example in which the second hologram element 651 includes theportions in which the interference fringe pitches are mutuallydifferent, a case in which the interference fringe pitches of the endportion 651 b are relatively sparser than the central portion 651 a andthe interference fringe pitches of the end portion 651 c are relativelydenser than the central portion 651 a can be exemplified. In this way,the following advantages can be obtained. When the interference fringepitches are different partially in this way, the interference fringepitches are preferably configured to continuously vary. Accordingly, itis possible to suppress the deterioration in the resolution occurringwhen the interference fringe pitches discontinuously vary.

However, by providing the portions in which the interference fringepitches are mutually different in the second hologram element 651, thereis a concern that the difference in the interference fringe pitches ofthe first hologram element 351 increases in some portions. When thedifference in the interference fringe pitches of the first hologramelement 351 increases, as described above, there is a concern that theangle width or the angle change of the diffraction angles occurring inthe first diffraction may not be sufficiently offset in the seconddiffraction.

In consideration of these facts, the interference fringe pitches of thefirst hologram element 351 are preferably set to be equal to or lessthan twice of the maximum value of the interference fringe pitches ofthe second hologram element 651 and to be equal to or greater than halfof the minimum value. When the interference fringe pitches are set inthis way, the function of offsetting, in the second diffraction, theangle width or the angle change of the diffraction angles occurring inthe first diffraction may not be said to be sufficient. However,occurrence of the deterioration in the resolution or the color deviationcan be suppressed further than when the first hologram element 351 isnot provided.

More preferably, the interference fringe pitches of the first hologramelement 351 are set to be between the maximum value and the minimumvalue of the interference fringe pitches of the second hologram element651. In such setting, a difference between the interference fringepitches of the first hologram element 351 and the second hologramelement 651 can be sufficiently small in the substantially entire regionof the second hologram element 651 even when there is the difference inthe interference fringe pitches in the second hologram element 651.Therefore, in the substantially entire region of the second hologramelement 651, the angle width or the angle change of the diffractionangles occurring in the first diffraction can be sufficiently offset.

In contrast, the interference fringe pitches of the first hologramelement 351 are preferably set to be the same as the interference fringepitches in the central portion 651 a of the second hologram element 651.Accordingly, for example, when the interference fringe pitches of thesecond hologram element 651 are distributed with a constant widthfocusing on the interference fringe pitches in the central portion 651a, the angle width or the angle change of the diffraction anglesoccurring in the first diffraction can be more sufficiently offset inthe substantially entire region of the second hologram element 651.

Further, in the second hologram element 651 in which the interferencefringe pitches are set in this way, the angle width or the angle changeof the diffraction angles in the central portion 651 a is relativelymost easily offset and the deterioration in the resolution or the colordeviation in the video light L5 diffracted and reflected in the centralportion 651 a is suppressed relatively most. The video light L5diffracted and reflected in the central portion 651 a is generallyconsidered to be light that contains information of relatively highimportance in the video and is easily viewed unconsciously by the eye EYof the observer. Accordingly, the deterioration in the resolution or thecolor deviation in the video light L5 diffracted and reflected in thecentral portion 651 a is sufficiently suppressed, and thus the videowith high quality can be viewed.

The interference fringe pitch (the diffraction grating period) of thesecond hologram element 651 refers to a value obtained in a scanningline SL that passes through the center of the amplitude of thesub-scanning in the vertical direction (the upper and lower directionsof FIG. 10) and is formed in the main scanning in the horizontaldirection (the right and left directions of FIG. 10) in the scanningrange of the video light L3 (corresponding to the scanning range of thevideo light L3 in the second hologram element 651 in the embodiment).

The central portion 651 a of the second hologram element 651 refers to aposition which is the center of the amplitude of the sub-scanning in thevertical direction and the center of the amplitude of the main scanningin the horizontal direction in the scanning range of the video light L3(corresponding to the scanning range of the video light L3 in the secondhologram element 651 in the embodiment).

In contrast, the interference fringe pitches of the first hologramelement 351 may be set to be the same as an average value of theinterference fringe pitches of the second hologram element 651. In suchsetting, a difference between the interference fringe pitches of thefirst hologram element 351 and the second hologram element 651 can besufficiently small in the substantially entire region of the secondhologram element 651 even when there is the difference in theinterference fringe pitches in the second hologram element 651.Therefore, in the substantially entire region of the second hologramelement 651, the angle width or the angle change of the diffractionangles occurring in the first diffraction can be more sufficientlyoffset.

As described above, the interference fringe pitches of the firsthologram element 351 are regulated based on the magnitude relation withthe interference fringe pitches of the second hologram element 651.However, in contrast to this, the interference fringe pitches of thesecond hologram element 651 may be regulated based on the interferencefringe pitches of the first hologram element 351.

For example, the interference fringe pitch of the second hologramelement 651 is preferably set to be included within a range equal to orgreater than 70% and equal to or less than 130% of the interferencefringe pitch of the first hologram element 351 and is more preferablyset to be included within a range equal to or greater than 90% and equalto or less than 110% of the interference fringe pitch of the firsthologram element 351. When the interference fringe pitch of the secondhologram element 651 is included within this range, the interferencefringe pitch of the second hologram element 651 is entered within arelatively narrow range focusing on the interference fringe pitch of thecentral portion 651 a. Accordingly, the angle width or the angle changeof the diffraction angles occurring in the first diffraction can beparticularly sufficiently offset in the substantially entire region ofthe second hologram element 651.

As a specific example, when green light with a wavelength of 515 nm isdiffracted and the density of the interference fringes of the firsthologram element 351 is 1550 per mm, the density of the interferencefringes of the central portion 651 a of the second hologram element 651is preferably 1550 per mm, and the densities of the interference fringesof the end portions 651 b and 651 c of the second hologram element 651are preferably equal to or greater than 1085 and equal to or less than2015 per mm and are more preferably equal to or greater than 1395 andequal to or less than 1705 per mm.

In this case, further more preferably, the densities of the interferencefringes of the end portions 651 b and 651 c of the second hologramelement 651 are considered to be equal to or greater than 1490 and equalto or less than 1700 per mm.

On the other hand, when blue light with a wavelength of 450 nm isdiffracted, the density of the interference fringes of the firsthologram element 351 is 1790 per mm and the above-described densities ofthe interference fringes of the second hologram element 651 may beaccordingly decided as described above.

Further, when red right with a wavelength of 630 nm is diffracted, thedensity of the interference fringes of the first hologram element 351 is1270 per mm and the above-described densities of the interferencefringes of the second hologram element 651 may be accordingly decided asdescribed above.

The foregoing calculation example is a calculation example when theangle of view is ±15 degrees right and left and video light is scannedso that a virtual image with a size corresponding to 60 inches can beviewed ahead 2.5 m. In this calculation example, the resolution of thevideo is assumed to be 720 P and the aspect ratio of the video isassumed to be 16:9.

In the image display device according to the invention, the resolutionis not particularly limited. For example, the resolution may be 1080 Por 2160 P. Further, the aspect ratio is not particularly limited either.For example, the aspect ratio may be 4:3 or 2.35:1.

In this way, even when the slight difference is present in theinterference fringe pitches between the first hologram element 351 andthe second hologram element 651, the angle width or the angle change ofthe diffraction angles occurring in the first diffraction can besufficiently offset in the second diffraction by setting the differencewithin the foregoing range, and thus the video can be affected as littleas possible. In other words, when the difference in the interferencefringe pitches between the first hologram element 351 and the secondhologram element 651 is within the foregoing range, the influence on thevideo can be suppressed so that the observer can rarely recognize theinfluence by undergoing the diffraction twice despite the fact that theangle width or the angle change of the diffraction angles occurs in thefirst diffraction.

In the embodiment, however, the video light L3 is assumed to bediffracted to be incident on the eye EY of the observer. Therefore, whenthe interference fringe pitches of the second hologram element 651 aredecided, it may be in some cases difficult to cause the advantages ofsuppressing the deterioration in the resolution and the color deviationto be compatible, as described above, by causing the video light L5 tobe reliably incident on the eye EY of the observer depending on, forexample, the size of the second hologram element 651, a distance betweenthe second hologram element 651 and the observer, and a positionalrelation between the second hologram element 651 and the imagegeneration unit 3.

In consideration of these cases, the reflection unit 6 according to theembodiment is configured such that the surface located on the side ofthe observer is the concave surface 611. That is, the surface shape ofthe second hologram element 651 on the incident side of the video lightL3 is also formed as a concave surface. The surface shape of the secondhologram element 651 on the incident side of the video light L3 may beat least a concave surface in a direction (that is, a direction verticalto the extension direction of the grating pattern of the diffractiongrating) vertical to the diffraction grating of the second hologramelement 651. By providing the second hologram element 651 on the concavesurface 611, the concave surface 611 operates to reinforce the functionof offsetting at least part of the angle width of the diffraction anglesoccurring in the first diffraction in the second diffraction in thesecond hologram element 651. That is, as described above, the secondhologram element 651 condenses the video light L5 generated by thediffraction in the second hologram element 651 toward the eye EY of theobserver. However, there is a background in which it is difficult tofreely select the diffraction angle when the second hologram element 651is designed based on the restriction of the above-described interferencefringe pitches, specifically, the restriction in which the difference inthe interference fringe pitches is not too large in the second hologramelement 651.

In contrast, when the second hologram element 651 is provided on theconcave surface 611 using the reflection unit 6 including the concavesurface 611, as in the embodiment, the concave surface 611 has theequivalent function to a condensing lens. Therefore, the function ofcondensing the video light L5 toward the eye EY is reinforced. As aresult, the observer can view the video with a large angle of view andhigh quality. As the condensing function is reinforced, the differencein the interference fringe pitches in the second hologram element 651may not be large to the extent. That is, even when the angle width orthe angle change of the diffraction angles occurring in the firstdiffraction may not be sufficiently offset in the second hologramelement 651, at least part of the deficit of the offset can besupplemented by the concave surface 611.

Accordingly, the reflection unit 6 may be formed in a flat shape with aflat surface, but is preferably considered to include the concavesurface 611 as in the embodiment. Accordingly, it is possible to morereliably suppress the deterioration in the resolution or the colordeviation of the video caused due to the angle width or the angle changeof the diffraction angles.

Here, for example, a calculation example in which conditions of thediffraction grating necessary to correct the position deviation or thecolor deviation of the video light L5 formed as the image on the retinaof the observer are compared between when the reflection unit 6 has theconcave surface 611 and when the reflection unit 6 has a flat surfaceinstead of the concave surface 611 will be described.

When the flat surface is used instead of the concave surface 611 and adensity at which the interference fringes provided in a central portionof the flat surface of the reflection unit 6 are formed is assumed to be1550 per mm, it is necessary for a formation density to have a width ina range in which the minimum value is 980 per mm and the maximum valueis 2200 per mm on the entire flat surface in the calculation. That is,the maximum difference in the formation density in the flat surface is1220 per mm. The width of the formation density is based on the factthat there is a difference in an angle at which the video light L3 isdiffracted in the flat surface of the reflection unit 6, as describedabove.

In contrast, in the embodiment, when the formation density of theinterference fringes provided in the central portion of the concavesurface 611 of the reflection unit 6 is assumed to be 1550 per mm, it isnecessary for the formation density to have a width in a range in whichthe minimum value is 1490 per mm and the maximum value is 1700 per mm onthe entire concave surface 611 in the calculation. That is, the maximumdifference in the formation density in the flat surface is suppressed to210 per mm.

In the calculation example, by providing the concave surface 611 in thereflection unit 6, it is provided that the difference in theinterference fringe pitches formed in the reflection unit 6 issuppressed to be small. By suppressing the difference in theinterference fringe pitches formed in the reflection unit 6 in this way,it is possible to achieve high quality in the entire video as well asthe central portion of the video. This is because the difference in theinterference fringe pitches between the first hologram element 351 andthe second hologram element 651 when the interference fringe pitches areconstant is suppressed. Therefore, the advantage of correcting theposition deviation or the color deviation of the video light L5 formedas the image on the retina of the observer is enhanced in the entirereflection unit 6 (the entire video), and thus the high quality isachieved.

The shape of the concave surface 611 is not particularly limited. Forexample, the shape of the concave surface 611 may be a free curvedsurface (aspheric surface), a spherical surface, a hyperboloid, or aparabolic surface.

The extension direction of the interference fringes of the firsthologram element 351 is preferably parallel to the extension directionof the interference fringes of the second hologram element 651.Specifically, in the case of FIG. 3, the extension direction of theinterference fringes of the first hologram element 351 and the extensiondirection of the interference fringes of the second hologram element 651are preferably directions perpendicular to the sheet surface.Accordingly, a relation between the diffraction direction (the emissiondirection of the diffracted light) of the incident light in the firstdiffraction and the diffraction direction (the emission direction of thediffracted light) of the incident light in the second diffraction is thesame. Accordingly, the angle width or the angle change of thediffraction angles occurring in the first diffraction is offset morereliably than in the second diffraction.

From the viewpoint of obtaining at least the foregoing advantages, theextension direction of the interference fringes of the first hologramelement 351 may not be necessarily parallel to the extension directionof the interference fringes of the second hologram element 651. Forexample, when the extension direction of the interference fringes of thefirst hologram element 351 is parallel to the extension direction of theinterference fringes of the second hologram element 651, the foregoingadvantages can be obtained even in a rotation state of the secondhologram element 651 at any rotation angle using the axis orthogonal tothe extension direction as a rotation axis, for example, a rotationstate (a state in which a so-called “blast” is received) of the secondhologram element 651 using the horizontal axis (the X axis) as therotation axis.

The parallel state includes a deviation state of the completeparallelism at an angle width of ±2°.

The extension direction of the interference fringes (diffractiongrating) of the second hologram element 651 is preferably orthogonal tothe direction of the main scanning of the video light L3, that is, thehorizontal direction. As described above, the angle width of thediffraction angles occurring in the first diffraction is the angle widthwhich is the width of the video light L3 in the direction of the mainscanning. Therefore, it is necessary to dispose the interference fringesso that the diffraction occurs to offset the angle width in the seconddiffraction. Accordingly, by matching the extension direction of theinterference fringes of the second hologram element 651 to the directionorthogonal to the direction of the main scanning of the video light L3,the angle width of the diffraction angles occurring in the firstdiffraction can be more reliably offset in the second diffraction.

The orthogonal state includes a deviation state of the completeorthogonality at an angle width of ±2°.

In the image display device 1 according to the embodiment, as describedabove, the video is formed using the three pieces of color light, thered light, the green light, and the blue light. Accordingly, in each ofthe first hologram element 351 and the second hologram element 651, theinterference fringes for the red light, the interference fringes of thegreen light, and the interference fringes of the blue light aresuperimposed (multiplexed) to be formed. Therefore, each of the firsthologram element 351 and the second hologram element 651 canindividually diffract and reflect the red light, the green light, andthe blue light at an optimum angle. As a result, for each of the videolight L5 formed from the red light, the video light L5 formed from thegreen light, and the video light L5 formed from the blue light, it ispossible to suppress occurrence of the angle width or the angle changeof the diffraction angles and it is possible to obtain the full-colorvideo in which the deterioration of the resolution or the colordeviation is suppressed.

Accordingly, as described above, the magnitude relation of theinterference fringe pitches between the first hologram element 351 andthe second hologram element 651 or the magnitude relation of theinterference fringe pitches in the second hologram element 651 isindividually and mutually independently established for the interferencefringes for the red light, the interference fringes for the green light,and the interference fringes for the blue light. Therefore, for example,for the red light, the angle width or the angle change of thediffraction angles occurring in the first diffraction is offset at leastpartially in the second diffraction. Similarly, for the green light, theangle width or the angle change of the diffraction angles occurring inthe first diffraction is offset at least partially in the seconddiffraction. Further, for the blue light, the angle width or the anglechange of the diffraction angles occurring in the first diffraction isoffset at least partially in the second diffraction.

To manufacture the first hologram element 351 or the second hologramelement 651 described above, for example, any of various manufacturingmethods such as an adhesion exposure scheme, a one-light fluxinterference scheme, a two-light flux interference scheme, and acollinear scheme is used.

To superimpose the interference fringes suitable for the plurality ofkinds of light with the different wavelengths described above, exposuremay be performed using a plurality of kinds of light with differentwavelengths at the time of the exposure of an object to be processed inthe manufacturing method.

Examples of the method of manufacturing the first hologram element 351included in the light reflection unit 114 include a method of formingthe first hologram element 351 in advance and subsequently attaching thefirst hologram element 351 to the plate-shaped member 113 and a methodof disposing an object to be processed for forming a hologram element ona substrate with a light transmissive property, performing an exposureprocess or the like on the object to be processed, and subsequentlydisposing the hologram element in the support portion 15 using thesubstrate as the above-described plate-shaped member 113.

In the hologram element, when light with a wavelength used at the timeof the manufacturing of the hologram element is incident, particularly,high diffraction efficiency is achieved and diffraction rarely occurs inthe light with a wavelength other than the wavelength (wavelengthselectivity is high). Accordingly, even when interference fringes forlight with a different wavelength are superimposed on one hologramlayer, it can be easy to maintain independence of the interferencefringes for each piece of light and it is possible to suppressoccurrence of the angle width or the angle change of the diffractionangles in each of the red light, the green light, and the blue light.

In the image display device 1, another color light may be added besidesthe red light, the green light, and the blue light. In contrast, lightof colors less than three colors, that is, only light of one color orlight of two colors, may be used.

In the embodiment, the light reflection unit 114 of the light scanningunit 36 is configured by the light diffraction unit 35. Therefore, thelight diffraction unit 35 and the light scanning unit 36 are integrated,and thus it can be easy to handle both of the light diffraction unit 35and the light scanning unit 36 and the volume of the image generationunit 3 can be decreased further than when the light diffraction unit 35and the light scanning unit 36 are mutually independently provided. As aresult, it can be easy to assemble (manufacture) the image displaydevice 1 and it is possible to miniaturize the image display device 1and realize low cost.

The video light L1 emitted from the video light generation unit 31 isprojected to a specific position of the light diffraction unit 35regardless of the content of the video. In other words, by configuringthe light diffraction unit 35 as the light reflection unit 114 of thelight scanning unit 36, an area necessary for the light diffraction unit35 can be suppressed to be small further than, for example, when thelight diffraction unit 35 is disposed between the light scanning unit 36and the expansion optical system 4. Accordingly, according to theembodiment, the light diffraction unit 35 with the small area can beused, and thus it is possible to miniaturize the image display device 1and realize low cost.

In the embodiment, the light diffraction unit 35 sways (reciprocates androtates) as the light reflection unit 114. Therefore, the posture of thefirst hologram element 351 also changes with respect to the incidentdirection of the video light L1. As a result, the advantage ofoffsetting the angle width of the diffraction angles occurring in thefirst diffraction is enhanced further than when the light diffractionunit 35 does not sway. This is more prominent when the video light isprojected toward the end portions 651 b and 651 c than when the videolight is projected toward the central portion 651 a in the secondhologram element 651.

When the light diffraction unit 35 is formed as a surface relief typediffraction grating or a surface relief hologram element, an inclinedsurface of a surface relief groove reflects light in a mirror reflectionmanner. Therefore, it is desirable to appropriately set the shape of thesurface relief groove so that the diffracted light diffracted in apredetermined direction based on the shape of the surface relief grooveis oriented toward the light scanning unit 36.

Incidentally, in the image display device 1 according to the embodiment,as described above, the reflective diffraction element is swayed by thelight scanning unit 36 including the light diffraction unit 35, so thatthe video light L3 which is the diffracted light is scanned and thevideo is formed.

FIGS. 12 to 14 are diagrams illustrating an example in which the videolight scanned by the light scanning unit is projected to the reflectionunit and is formed on a retina.

In the example illustrated in FIGS. 12 to 14, the posture of the entirelight scanning unit 36 is maintained so that a normal line N of thelight reflection unit 114 is inclined by only angle α with respect tothe light axis of the video light L1. When the light scanning unit 36 isdriven to sway the light reflection unit 114 (the light diffraction unit35) in this state, the diffracted light is emitted with a predeterminedangle width, specifically, within a range interposed between video lightL30 and video light L31. As a result, the video light L30 and the videolight L31 are projected to the reflection unit 6, and thus thediffracted light of the video light L30 and the diffracted light of thevideo light L31 are respectively formed as video light L50 and videolight L51 on the retina of the eye EY of the observer.

In this example, an angle β₀ is assumed to be an angle formed by thelight axis of the video light L1 and the light axis of the video lightL30 and an angle β₁ is assumed to be an angle formed by the light axisof the video light L1 and the light axis of the video light L31 (here,β₀<β₁).

Here, light contributing to the video formed in the same principle asthe above-described principle in the video light L1 is only the lightincident on the light reflection unit 114 (the light diffraction unit35). However, since the video light L1 projected toward the lightscanning unit 36 is projected with an angle width to some extent, theentire video light L1 is not diffracted in the light diffraction unit35. Part of the video light L1 is reflected in the surface of the lightdiffraction unit 35 or the surface of the plate-shaped member 113 or thesurface of each portion of the light scanning unit 36. The video lightL1 reflected in this way is projected in an unintended direction, butpart of the video light L1 is incident as stray light on the retina.Such stray light results in deterioration in the visibility of the videoand deterioration in the image quality.

Of the drawings, FIG. 12 illustrates an example of a case in which partof the video light L1 arrives at the support portion 15 included in thelight scanning unit 36 when the video light L1 is projected toward thelight scanning unit 36.

As illustrated in FIG. 6, the support portion 15 is a portion thatsupports the movable mirror 11 and the posture is not changed regardlessof the swaying of the movable mirror 11. In the example of FIG. 12, theposture of the entire light scanning unit 36 is maintained so that thenormal line N of the surface of the light reflection unit 114 (the lightdiffraction unit 35) with respect to the light axis of the video lightL1 is inclined by the angle α. Therefore, the angle formed by the lightaxis of the video light L1 and the normal line of the surface of thesupport portion 15 is the angle α. As a result, when the video light L1is reflected from the surface of the support portion 15, reflected lightL7 is projected in a direction inclined by an angle 2α with respect tothe light axis of the video light L1.

Accordingly, it is necessary to configure the image display device 1 sothat the reflected light L7 emitted from the support portion 15 is notincident on the retina of the observer.

Specifically, in the example illustrated in FIG. 12, the image displaydevice 1 preferably satisfies formula [1] below.

2α<β₀  [1]

By configuring the image display device 1 so that such a condition issatisfied, it is possible to decrease a probability at which thereflected light L7 reaches an effective range of the second hologramelement 651, that is, a specific range of the second hologram element651 in which the light can be diffracted toward the eye EY of theobserver. In other words, it is possible to reduce a probability atwhich the reflected light L7 reflected from the portion such as thesupport portion 15 becomes stray light. As a result, it is possible tosuppress the deterioration in the visibility of the video.

When the stray light emitted from the support portion 15 is incident onthe retina, a time at which the stray light is continuously incident isoverwhelmingly longer than a time in which the video light L50 and thevideo light L51 are continuously incident. Therefore, the luminance ofthe stray light is considerably larger than the luminances of the videolight L50 and the video light L51. Therefore, from the viewpoint ofimproving the visibility of the video, it is very effective to satisfythe above-described condition and avoid the incidence of the straylight.

Formula [1] above can also be applied to portions other than the supportportion 15.

FIG. 13 illustrates an example of a case in which part of the videolight L1 arrives at the frame body 13 included in the light scanningunit 36 when the video light L1 is projected toward the light scanningunit 36.

As illustrated in FIG. 6, the frame body 13 is a portion that supportsthe movable mirror 11 and is the portion of which a posture is changedwith the swaying of the movable mirror 11, as illustrated in FIG. 13. Inthe example of FIG. 13, the posture of the entire light scanning unit 36is maintained so that the normal line N of the surface of the lightreflection unit 114 (the light diffraction unit 35) with respect to thelight axis of the video light L1 is inclined by the angle α. Therefore,the angle formed by the light axis of the video light L1 and the normalline of the surface of the frame body 13 is the angle α. As a result,when the video light L1 is reflected from the surface of the frame body13, the reflected light L7 is projected in a direction inclined by theangle 2α with respect to the light axis of the video light L1.

On the other hand, as described above, the frame body 13 sways with theswaying of the movable mirror 11, but the amplitude of the swaying isless than the amplitude of the swaying of the movable mirror 11. When γ₀is the amplitude of the swaying of the movable mirror 11 and n is amagnification of the amplitude of the swaying of the movable mirror 11with respect to the amplitude of the swaying of the frame body 13, theamplitude of the swaying of the frame body 13 is expressed as γ₀/n.

Accordingly, it is necessary to configure the image display device 1 sothat the reflected light L7 emitted from the frame body 13 is notincident on the retina of the observer.

Specifically, in the example illustrated in FIG. 13, the image displaydevice 1 preferably satisfies formula [2] below.

$\begin{matrix}{{{\frac{\gamma_{0}}{n} + {2\alpha}} < \beta_{0}}{\alpha < {\frac{\beta_{0}}{2} - \frac{\gamma_{0}}{2n}}}} & \lbrack 2\rbrack\end{matrix}$

By configuring the image display device 1 so that such a condition issatisfied, it is possible to decrease a probability at which thereflected light L7 reaches an effective range of the second hologramelement 651, that is, a specific range of the second hologram element651 in which the light can be diffracted toward the eye EY of theobserver. In other words, it is possible to reduce a probability atwhich the reflected light L7 reflected from the portion such as theframe body 13 becomes stray light. As a result, it is possible tosuppress the deterioration in the visibility of the video.

When the stray light emitted from the frame body 13 is incident on theretina, the amplitude of the swaying of the frame body 13 is less thanthe amplitude of the swaying of the movable mirror 11. Therefore, theluminance of the stray light emitted from the frame body 13 is largerthan the luminances of the video light L50 and the video light L51.Therefore, from the viewpoint of improving the visibility of the video,it is very effective to satisfy the above-described condition and avoidthe incidence of the stray light.

FIG. 14 illustrates an example of a case in which part of the videolight L1 arrives at the surface of the light diffraction unit 35configuring the light reflection unit 114 of the light scanning unit 36when the video light L1 is projected toward the light scanning unit 36.

The first hologram element 351 configuring the light diffraction unit 35has the interference fringes therein and functions as a reflectivediffraction element. As illustrated in FIG. 6, the light diffractionunit 35 configures the light reflection unit 114 and is a portion ofwhich a posture is changed with the swaying of the movable mirror 11. Inthe example of FIG. 14, the posture of the entire light scanning unit 36is maintained so that the normal line N of the surface of the lightreflection unit 114, that is, the normal line of the surface of thelight diffraction unit 35, with respect to the light axis of the videolight L1 is inclined by the angle α. Therefore, the angle formed by thelight axis of the video light L1 and the normal line of the surface ofthe light diffraction unit 35 is the angle α. As a result, when thevideo light L1 is reflected from the surface of the light diffractionunit 35, the reflected light L7 (so-called 0th-order light) is projectedin a direction inclined by the angle 2α with respect to the light axisof the video light L1.

On the other hand, when γ₀ is the amplitude of the swaying of themovable mirror 11, an angle formed by the light axis of the video lightL1 and the light axis of the reflected light L7 is γ₀+206 when thereflected light L7 becomes stray light.

Accordingly, it is necessary to configure the image display device 1 sothat the reflected light L7 emitted from the surface of the lightdiffraction unit 35 is not incident on the retina of the observer.

Specifically, in the example illustrated in FIG. 14, the image displaydevice 1 preferably satisfies formula [3] below.

$\begin{matrix}{{{\gamma_{0} + {2\alpha}} < \beta_{0}}{\alpha < \frac{\beta_{0} - \gamma_{0}}{2}}} & \lbrack 3\rbrack\end{matrix}$

By configuring the image display device 1 so that such a condition issatisfied, it is possible to decrease a probability at which thereflected light L7 reaches an effective range of the second hologramelement 651, that is, a specific range of the second hologram element651 in which the light can be diffracted toward the eye EY of theobserver. In other words, it is possible to reduce a probability atwhich the reflected light L7 reflected from the surface of the lightdiffraction unit 35 becomes stray light. As a result, it is possible tosuppress the deterioration in the visibility of the video.

Second Embodiment

Next, a second embodiment of the image display device according to theinvention will be described.

FIGS. 15A and 15B are diagrams schematically illustrating aconfiguration of the second embodiment of the image display deviceaccording to the invention.

Hereinafter, the second embodiment will be described. In the followingdescription, differences from the above-described first embodiment willbe mainly described and the description of the same portions will beomitted. In the drawings, the same reference numerals are given to thesame portions as those of the above-described embodiment.

An image display device 1 according to the second embodiment is the sameas the image display device 1 according to the first embodiment exceptthat the configurations of the first hologram element 351 and the secondhologram element 651 are different.

That is, in each of the first hologram element 351 and the secondhologram element 651 according to the above-described first embodiment,interference fringes for red light, interference fringes for greenlight, and interference fringes for blue light are superimposed(multiplexed) to be formed at different pitches in a one-layeredhologram layer so that three pieces of color light, the red light, thegreen light, and the blue light, are individually diffracted.

However, as illustrated in FIGS. 15A and 15B, the first hologram element351 according to the embodiment is configured as a laminate in which ahologram layer 351R diffracting the red light, a hologram layer 351Gdiffracting the green light, and a hologram layer 351B diffracting theblue light are stacked.

Similarly, as illustrated in FIGS. 15A and 15B, the second hologramelement 651 according to the embodiment is configured as a laminate inwhich a hologram layer 651R diffracting the red light, a hologram layer651G diffracting the green light, and a hologram layer 651B diffractingthe blue light are stacked.

In the embodiment, since the interference fringes for the red light, theinterference fringes for the green light, and the interference fringesfor the blue light are formed in the mutually different hologram layersin this way, deterioration of the diffraction efficiency caused due tothe superimposition of the interference fringes is suppressed.Therefore, in the embodiment, it is possible to improve the diffractionefficiency of each of the first hologram element 351 and the secondhologram element 651.

The magnitude relation of the interference fringe pitches between thefirst hologram element 351 and the second hologram element 651 or themagnitude relation of the interference fringe pitches in the secondhologram element 651, as described in the first embodiment, isindividually and mutually independently established for the interferencefringes for the red light, the interference fringes for the green light,and the interference fringes for the blue light in the embodiment.Therefore, for example, for the red light, the angle width or the anglechange of the diffraction angles occurring in the first diffraction isoffset at least partially in the second diffraction. Similarly, for thegreen light, the angle width or the angle change of the diffractionangles occurring in the first diffraction is offset at least partiallyin the second diffraction. Further, for the blue light, the angle widthor the angle change of the diffraction angles occurring in the firstdiffraction is offset at least partially in the second diffraction.

The stack order of the hologram layers 351R, 351G, and 351B and thestack order of the hologram layers 651R, 651G, and 651B are not limitedto the stack orders illustrated in FIGS. 15A and 15B.

In the above-described second embodiment, it is also possible to obtainthe same operations and advantages as those of the first embodiment.

Third Embodiment

Next, a third embodiment of the image display device according to theinvention will be described.

FIG. 16 is a diagram schematically illustrating an overall configurationof a head-up display of the third embodiment including the image displaydevice according to the invention.

Hereinafter, the third embodiment will be described. In the followingdescription, differences from the above-described first and secondembodiments will be mainly described and the description of the sameportions will be omitted. In the drawings, the same reference numeralsare given to the same portions as those of the above-describedembodiments.

An image display device 1 according to the third embodiment is the sameas the image display device 1 according to the first and secondembodiments except that the image display device 1 is included in ahead-up display 10′ mounted on a ceiling portion of an automobile foruse.

That is, the image display device 1 according to the third embodiment ismounted on a ceiling portion CE of an automobile CA for use and causesan observer to view a virtual image and an outside image in asuperimposition state.

As illustrated in FIG. 16, the image display device 1 includes a lightsource unit UT including an image generation unit 3 and an expansionoptical system 4, a reflection unit 6, and a frame 2′ connecting thelight source unit UT to the reflection unit 6.

In the embodiment, a case in which the light source unit UT, the frame2′, and the reflection unit 6 are mounted on the ceiling portion CE ofthe automobile CA will be exemplified. The light source unit UT, theframe 2′, and the reflection unit 6 may be mounted on a dashboard of theautomobile CA or some of the configuration may be fixed to a frontwindow FW. Further, the head-up display 10′ may be mounted not only onan automobile but also on any of various moving objects such as anairplane, a ship, a construction machinery, a heavy machinery, atwo-wheeled vehicle, a bicycle, and a space ship.

Hereinafter, the units of the image display device 1 according to theembodiment will be sequentially described in detail.

The light source unit UT may be fixed to the ceiling portion CE inaccordance with any method. For example, the light source unit UT isfixed in accordance with a method of mounting the unit on a sun visorusing a band, a clip, or the like.

The frame 2′ includes, for example, a pair of long members, and thus thelight source unit UT and the reflection unit 6 are fixed by connectingboth ends of the light source unit UT and the reflection unit 6 in the Xaxis direction.

The light source unit UT includes the image generation unit 3 and theexpansion optical system 4, and thus the video light L3 is emitted fromthe expansion optical system 4 to the reflection unit 6. Then, the videolight L5 diffracted and reflected by the reflection unit 6 is formed asan image on the eye EY of the observer.

On the other hand, the reflection unit 6 according to the embodimentalso has a function of transmitting outside light L6 oriented from theoutside of the reflection unit 6 to the eye EY of the observer at thetime of use. That is, the reflection unit 6 has a function of reflectingthe video light L3 from the light source unit UT and transmitting theoutside light L6 oriented from the outside of the automobile CA to theeye EY of the observer via the front window FW at the time of use.Accordingly, the observer can view the outside image and simultaneouslyview a virtual image (image) formed by the video light L5. That is, thesee-through head-up display can be realized.

In the third embodiment, it is also possible to obtain the sameoperations and advantages as those of the first and second embodiments.

That is, in the image display device 1 according to the embodiment, atleast part of the angle width or the angle change of the diffractionangles occurring in the first diffraction can be offset in the seconddiffraction. Accordingly, the observer can view the high-quality videoin which the deterioration in the resolution or the color deviation issufficiently suppressed.

The image display device according to the invention has been describedabove based on the illustrated embodiments, but the invention is notlimited thereto.

For example, in the image display device according to the invention, theconfiguration of each unit can be substituted with any configurationhaving the same function and any configuration can also be added.

Embodiments of the image display device according to the invention arenot limited to the head-mounted display or the head-up display describedabove, but any image display device can be included as long as the imagedisplay device has a display principle of a retina scanning scheme.

The entire disclosure of Japanese Patent Application No. 2015-038583,filed Feb. 27, 2015 is expressly incorporated by reference herein.

What is claimed is:
 1. An image display device comprising: a video lightgeneration unit that generates video light modulated based on a videosignal; a first diffraction optical element that diffracts the videolight emitted from the video light generation unit; a light scanner thatincludes a light reflection unit and spatially scans the video light byreflecting the video light in the light reflection unit; and a seconddiffraction optical element that diffracts the incident video light whenthe video light scanned by the light scanner is incident, wherein thelight reflection unit is configured by the first diffraction opticalelement.
 2. The image display device according to claim 1, wherein inthe second diffraction optical element, a surface shape on an incidentside of the video light is a concave surface in a directionperpendicular to a diffraction grating of the second diffraction opticalelement.
 3. The image display device according to claim 1, wherein thelight scanner performs main scanning of the video light in a firstdirection and performs sub-scanning of the video light in a seconddirection orthogonal to the first direction, wherein in the firstdiffraction optical element, a diffraction grating period is constant,and wherein the second diffraction optical element has portions in whichthe diffraction grating period is mutually different from a diffractiongrating period on a scanning line of the main scanning passing throughthe center of an amplitude of the sub-scanning of the video lightincident on the second diffraction optical element.
 4. The image displaydevice according to claim 3, wherein the diffraction grating period ofthe first diffraction optical element is between a maximum value and aminimum value of the diffraction grating period on the scanning line ofthe main scanning passing through the center of the amplitude of thesub-scanning of the video light incident on the second diffractionoptical element.
 5. The image display device according to claim 3,wherein the diffraction grating period of the first diffraction opticalelement is the same as a diffraction grating period at a position of thecenter of an amplitude of the main scanning and on the scanning line ofthe main scanning passing through the center of the amplitude of thesub-scanning of the video light incident on the second diffractionoptical element.
 6. The image display device according to claim 3,wherein the diffraction grating period of the first diffraction opticalelement is the same as an average value of the diffraction gratingperiod on the scanning line of the main scanning passing through thecenter of the amplitude of the sub-scanning of the video light incidenton the second diffraction optical element.
 7. The image display deviceaccording to claim 3, wherein an extension direction of the diffractiongrating of the second diffract ion optical element is orthogonal to thefirst direction.
 8. The image display device according to claim 1,wherein the light scanner includes a driving system that reciprocatesand rotates the light reflection unit, and wherein when γ₀ is anamplitude of the reciprocation and the rotation, α is an angle formed byan optical axis of the video light and a normal line of the lightreflection unit when the light reflection unit does not rotate, and β₀is a minimum value of an angle formed by the light axis of the videolight incident on the light reflection unit when the light reflectionunit reciprocates and rotates and the light axis of the video lightemitted from the light reflection unit, a formula below is satisfied.$\alpha < \frac{\beta_{0} - \gamma_{0}}{2}$
 9. The image display deviceaccording to claim 1, further comprising: a pupil expansion opticalsystem that is provided on a light path between the light scanner andthe second diffraction optical element.